The present invention relates to an automatic compensation circuit and method for use in a digital switching system.
As digital telephone switching systems are being developed to replace existing analog telephone switching systems, a typical implementation utilizes pulse code modulated (PCM) switching techniques.
In order to convert analog signals into a PCM format, the analog signal is first converted to a pulse amplitude modulated (PAM) format where each analog signal is sampled at 8 KHz. A plurality of PAM samples are multiplexed into time frames of 24 voice channels recurring at a 125 microsecond (.mu.s) rate. The PAM samples are converted to PCM format and then the PCM samples can be further multiplexed by known techniques to provide improved switching capabilities.
Typically, a PAM input signal is converted to a series of PCM serial samples utilizing, for example, a successive approximation register and digital to analog converter to convert the PAM samples into corresponding PCM samples.
In order to operate with digital switching systems, it is generally required that the input signal have no DC offset value (e.g., the average value of the pulse amplitude modulated signal is zero) since a DC offset level can affect the idle channel performance of the system. In particular, for an idle channel, a DC offset provides a bias which can increase the idle channel noise, decrease inter-channel crosstalk coupling loss, or both. An analog information signal, due to earlier AC coupling, has an average value of zero. However, an offset DC level is often found in digital switching systems because of the offset voltages in operational amplifiers used in active filters, offset voltages in sampling switches and the like. These offsets will typically be different on the different channels on a PAM bus.
Therefore, it would be desirable to provide improved circuitry which could compensate for offsets in DC levels for each channel of a PAM input signal.
In view of the above background, it is an objective of the present invention to provide an improved automatic compensation circuit for use in a digital switching system.